Preheat oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan.

Prepare the rhubarb swirl: In a medium bowl, combine chopped rhubarb, 1/4 cup granulated sugar, cornstarch, and 1 teaspoon cinnamon. Mix well and set aside.

In a large bowl, whisk together flour, baking soda, salt, and 1 teaspoon cinnamon. In a separate bowl, cream together softened butter and 1 1/2 cups granulated sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla extract.

Add half of the dry ingredients to the wet ingredients and mix until just combined. Pour in the buttermilk and mix until smooth. Add the remaining dry ingredients and mix until just combined, being careful not to overmix.

Spread half of the batter into the prepared loaf pan. Spoon the rhubarb mixture evenly over the batter. Top with the remaining batter, spreading it to cover the rhubarb. Swirl gently with a knife or skewer if desired.

Bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out clean. Let cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Prepare the glaze: Whisk together powdered sugar and 2 tablespoons of milk until smooth. Add more milk, 1 teaspoon at a time, if needed to reach desired drizzling consistency. Drizzle over cooled bread.